110 



2. GENERAL. 



The genus Flabelluni is characterised among the Turbin- 

 olidae by having a well-defined '"■ epitheca," and together 

 with this the absence of any structure, which may be termed 

 a " theca," The genus Antillia shows typically the difference 

 between " theca " and " epitheca." The first is the wall — be 

 it formed as a basal deposit or by thickenings of the septal 

 sides — surrounding the digestive cavity of the polyp, while 

 the latter closes off such parts of the anemone as may lie 

 outside the " theca " from the external medium, i.e., the sea- 

 water. According to this definition the " theca " should be 

 covered on and formed from both sides by the tissues of the 

 polyp, while the " epitheca " is only so covered on and formed 

 from one, i.e , the inner side. Accepting the fact that the 

 skeleton lies completely external to the polyp — a reality not 

 within the knowledge of the proposers of the terms — the 

 above forms a reliable and indeed the only distinction, unless 

 it be subsequently shown that the two are formed essentially 

 differently from one another. 



" Costae " correspond to the septa, and are their continua- 

 tions outside the theca. The theca is not formed before the 

 septa, but may be built up pan passiL with their formation. 

 More often the septa are formed first, and from the beginning 

 project above the theca, i.e., are " exsert."' In Antillia the 

 edges of the " costae " are fused with the epitheca, but in most 

 forms there is no such epitheca, and they are hence covered 

 over outside by the soft tissues. Where epitheca alone is 

 present — as in Flabelluni — there can be no costae, although 

 raised ribs of the epitheca may simulate them. The term 

 " exsert " applied to the septa also bears relation to the 

 presence of a theca, and cannot properly be applied where 

 none such is present. 



The only case, where there can in practice be any doubt 

 between theca and epitheca, is where a theca without costae 

 has been formed. In such a case, if the edge-zone of the 

 polyp — that part which lies external to the theca — withdraws 

 completely, there may actually be no tissues external to the 

 wall. The latter, if a theca, always shows in section a definite 

 dark line along its centre, and in a living polyp some of the 

 top or upper part of the wall would still necessarily be covered 

 by an edge-zone. There would further be no distinction 

 between the inner and outer sides of such a theca. A possible 

 extreme case would be where the calicoblastic ectoderm of 

 the edge-zone in its retreat deposited a special coating of 

 carbonate of lime. The glassy appearance of DeamopJiylluin 

 in its lower parts indicates the downward extent of the edge- 

 zone, and may be due to such a deposit. This, however, does 



